Apparatus for detecting malfunction in fuel evaporative prurge system

An apparatus for detecting a malfunction in an evaporated fuel purge system provided in an internal combustion engine, comprising:

• purge means for performing alternately a purging mode and a purge cutting mode, fuel vapor from a fuel tank being adsorbed in an adsorbent in a canister when said purge cutting mode is performed by said purge means, and when said purging mode is performed by said purge means the adsorbed fuel vapor in the adsorbent being purged into an intake passage of the internal combustion engine;
• detection means for detecting an air-fuel ratio of the fuel vapor being purged into the intake passage so that a change in the air-fuel ratio of the fuel vapor, from a time when the purge cutting mode is performed to a time when the purging mode is performed by said purge means immediately after said purge cutting mode has been performed, is detected;
• malfunction discrimination means for determining whether there is a malfunction in the evaporated fuel purge system, by comparing said change in said detected air fuel ratio of the fuel vapor detected by said detection means with a predetermined value;
• fuel vapor detection means for detecting a fuel vapor pressure in the fuel tank;
• and
• purge cut time varying means, coupled to said purge means, said purge cut time varying means determining a purge cut time, during which said purge cutting mode is continuously performed by said purging means, said purge cut time being varied by said purge cut time varying means in response to said fuel vapor pressure in the fuel tank detected by said fuel vapor detection means, wherein the purge cut time is determined based on the fuel vapor pressure of the fuel tank detected by said fuel vapor detection means.